In a device of this kind known from EP-A-0 199 059, the light source and the receiver pointing into the detection zone at the storage drum are provided in a bracket of the housing of the yarn feeder and distant from the surface of the storage drum. Below the detection zone a glass plate is inserted into the surface of the storage drum. At the lower side of said glass plate a full plane reflective coating is provided. The yarn windings wound onto the storage drum in circumferential direction are pushed axially forwards so that each yarn winding passes the detection zone lateral to its longitudinal direction. The device serves as a sensor for the detection of the axial position of at least the first winding of the yarn supply on the storage drum and is responding to the movement, absence or presence of the yarn with a signal which, e.g., is used for the control of the rotational drive of the yarn feeder. The receiver exclusively registers the shade of the yarn or the variation of the light intensity as soon as the yarn is interrupting the light beams directed towards the reflector or coming from the reflector. Independent from the design, only a relatively weak modulation of the signal can be derived from the shadowing, particularly in case of thin yarns, such that in order to generate a useful and clear active signal relatively high efforts are necessary for the evaluation, amplification and discrimination. The device is vulnerable in case of dirt, disturbing light or unavoidable lint or lint bundles. In addition, the frequency of necessary cleaning--or maintenance--cycles is undesirably high.
It is, nevertheless, for similar devices known to provide a diaphragm and/or lenses for an opto-electronic yarn detection between the yarn and the receiver and/or between the light source and the yarn. However, that measure cannot significantly improve the response behavior of the device and does not have a considerable influence on the vulnerability against contamination.
It is an object of the invention to create a device of the kind as disclosed which is apt to derive strong and useful effective signals from the passage of the yarn through or the presence or absence of the yarn in the detection zone, which is relatively insensitive for contaminations, lint and disturbing light.